In the fabrication of semiconductor devices, it is frequently necessary to etch a substrate surface to form a trench or a via. To accomplish this, several different methods have been developed including barrel type etching, reactive ion etching and ion beam etching.
Barrel type etching utilizes a cylindrical tube that forms a barrel shaped container. The container is wrapped with a wire coil that is capable of emitting radio frequency energy. Gas is supplied to the container and radio frequency energy, emitted through the walls of the container, excites the gas into a plasma state which is evidenced by free electrons and ions. The surface to be etched is placed inside the container and bombarded by the plasma. There is no direction to the plasma, and, therefore, it attacks the surface equally in every direction. The surface is removed from the container after being etched to the proper depth. A primary problem with the barrel type etcher is the random directionality of the plasma, which allows no uniformity in the etching. The typical barrel etch thus resembles a dished out trench.
Reactive ion etching allows more directionality to the plasma. In this type of etching, two parallel plates are electrically connected to a RF energy source. One plate is usually biased with a relatively high voltage (100-1000 volts) to accelerate the plasma toward a surface. Gas enters a container in which the plates are positioned, and the RF energy excites the gas to form plasma. Due to the applied voltage, the ions formed within the plasma obtain a directionality which is aimed at the surface to be etched. This etcher may damage the surface due to the acceleration of the ions caused by the high voltage. Lattice defects, impurity sites and thermal damage to the surface are typical of the reactive ion etcher.
A third technique typically used is ion beam etching. An ion beam etcher comprises a cylindrical container through which gases are excited into a ionized state by an electric filament. An electric potential is used to accelerate and aim the ions at the surface to be etched. The ion beam etcher uses almost pure physical etching, in contrast to the physical and chemical etching of the barrel and the reactive ion etchers, and can be related to a similar reaction obtained by throwing a billiard ball at a surface. The ion beam etcher is the most directional of the three etchers, but is also the most damaging to the surface. Thus, a need exists for a low cost method and apparatus for directional etching without resulting damage to the surfaces which are etched.